This invention is in the field of sewing machines; more particularly, it is concerned with an improved device for preventing rotation of the under thread bobbin of a lockstitch sewing machine except during times of thread demand in order to avoid spilling of the bobbin thread.
It frequently occurs that a freely spinning bobbin will release thread from the bobbin which will interfere with present or future stitch formation in the sewing machine. A freely spinning bobbin can result from a sudden machine stop from high speed operation, from a sudden pull on the bobbin thread from an under bed thread trimmer, or, simply, from a removal of work material from the sewing machine prior to cutting of needle and bobbin threads.
It is known in the prior art to apply to a bobbin containing lower thread, sufficient restraint to prevent overrunning of the bobbin and spilling of thread therefrom. Such a device is disclosed in U.S. Pat. No. 2,555,658, issued on June 5, 1951 to Ritter which utilized a leaf spring impinging on the bobbin flange to deter rotation thereof. Such a device is, however, effective at all times including times of thread demand, thereby having a sometimes adverse effect on bobbin thread tension.
It is also known to provide a pivoted lever in a bobbin case which is urged, for example, by a leaf spring into engagement with the periphery of a bobbin to restrain rotation thereof, but which lever is responsive to motion of bobbin thread caused by thread demand to pivot away from the bobbin flange periphery to allow free removal of thread therefrom. Such devices are disclosed in the U.S. Pat. Nos. 3,486,473, and 2,975,738, issued, respectively, on Dec. 30, 1969 and Mar. 21, 1961. However, these devices with their proliferation of fine parts including levers, pivot shafts, and spring loading means, are expensive and have been found to require very careful construction and adjustment.
What is required is a simple, inexpensive and fool proof device which may also be readily adapted for use in existing conventional arrangements.